1. Field of the Invention
The present invention relates generally to automatic gain control (hereinafter "AGC") circuits for use in a high frequency circuit device such as, a television receiver, CATV receiver and/or videotape recorder. Specifically, the invention relates to an AGC circuit using a PIN diode (P type-Intrinsic N type semiconductor diode) and a CATV receiver including such a circuit.
2. Description of the Related Art
The configuration and operation of a conventional AGC circuit will be described in conjunction with FIGS. 6 to 8. FIG. 6 is a block diagram showing the configuration of a tuner portion 2000 in a conventional bidirectional CATV receiver.
A terminal 10 is provided with high frequency signal inputs from a broadcasting station or a key station. The band of such high frequency signals is for example in the range from 52 MHz to 750 MHz.
A band pass filter 13 selectively passes only signals in the band of 52 MHz to 750 MHz among the input high frequency signals. An AGC circuit 44 receives a signal output from band pass filter 13 and controls the gain for the input high frequency signal based on the level of an AGC signal input to the terminal 15 of AGC circuit 44. A high frequency amplifier 16 receives and amplifies the high frequency signal from AGC circuit 44, and outputs the result to a mixer 17.
A local oscillator 18 oscillates a signal of a frequency shifted from the broadcasting frequency of a station chosen by the amount of intermediate frequency output to mixer 17. The intermediate frequency is for example 953 MHz.
Mixer 17 mixes the signal output from local oscillator 18 with the input high frequency signal.
The resulting mixed signal includes various frequency components. A band pass filter 19 limitatively passes signals in the band of for example 953 MHz among the signals output from mixer 17, and outputs a first intermediate frequency signal.
Then, an intermediate frequency amplifier 20 amplifies the first intermediate frequency signal, and a band pass filter 21 limits the band of the signal to pass.
A mixer 22 mixes the first intermediate frequency signal passed through band pass filter 21 and the signal transmitted by a local oscillator 23. The signal produced by mixing in a mixer 22 is received by a band pass filter 24, which in turn limits the band of the signal to pass, and outputs a second intermediate frequency signal of for example 43.5 MHz.
An intermediate frequency amplifier 25 amplifies the second intermediate frequency signal from band pass filter 24 for output to a terminal 26.
Terminal 11 receives a signal output to a broadcasting station or key station from the receiving side (hereinafter "up signal"). The up signal used has a frequency band lower than a signal received from the broadcasting station (hereinafter "down signal") and not higher than 42 MHz. Band pass filter 12 limits the band of the up signal to be applied to terminal 11 and outputs the band-limited signal to terminal 10.
Note that in the tuner in the bidirectional CATV receiver shown in FIG. 6, the gain of a received signal is controlled by AGC circuit 44. If high frequency amplifier 16 receives a signal in a level higher than specified, a signal output therefrom is distorted, which causes disturbance such as intermodulation and cross modulation. Therefore, AGC circuit 44 is usually provided preceding to high frequency amplifier 16.
The AGC signal applied to AGC circuit 44 is output from an AGC control circuit (not shown) which changes the level of the AGC signal to output depending upon the intensity of a signal received from intermediate frequency amplifier 25.
FIG. 7 is a circuit diagram showing the configuration of the conventional AGC circuit 44.
Herein, diodes D1 to D3 are PIN diodes.
FIG. 8 is a graph showing the change of high frequency resistance (rd) as the forward current of PIN diode (If) changes.
Now, referring to FIGS. 7 to 8, the operation of the conventional AGC circuit 44 will be detailed.
Referring to FIG. 7, terminal 1 is provided with a high frequency signal output from band pass filter 13 (see FIG. 6). The gain relative to the input high frequency signal is controlled depending on the gain of the AGC signal as will be described, and a high frequency signal is output from terminal 2.
The high frequency signal output from terminal 2 is transmitted to high frequency amplifier 16 shown in FIG. 6.
The voltage level of the AGC signal varies for example in the range from 0 to 8 V. The AGC signal is generated based on the intensity level of the intermediate frequency output, and the gain of the tuner is feedback-controlled in response to the AGC signal, so that the intensity of intermediate frequency output from the tuner is controlled to be constant.
The bias voltage supplied to the connection point of diode D2 and a capacitor C2 is voltage produced by dividing power supply voltage VB by resistors R1 and R2. The potential on the connection point of diode D2 and capacitor C2 is set lower than the potential on the connection point of diode D1 and a bias resistor R3 when the AGC voltage is maximized.
When the AGC signal level is at a maximum value, current is passed through a coil L2, diode D1 and bias resistor R3 by the voltage supplied by the AGC signal. The larger the current value, the smaller becomes the high frequency resistance of diode D1 as shown in FIG. 8, and the signal gain of the AGC circuit decreases as a result.
Note that the values of resistors R1 and R2 are set so that diodes D2 and D3 are both reversely biased when the AGC signal level is at its maximum value.
Then, as the AGC signal level decreases from its maximum value, current passed through diode D1 decreases as well, and the potential at the connection point of diode D1 and resistor R3 is lowered as a result. Current passed through diodes D2 and D3 increases accordingly. The high frequency resistance of diode D1 increases, and the high frequency resistance of diodes D2 and D3 decreases.
As a result, the high frequency resistance of PIN diode D1 disposed along the direction of the transmission of high frequency signals increases, while the high frequency resistance of PIN diodes D2 and D3 disposed crossing the direction of transmission decreases, resulting in an increase in the attenuation amount of the AGC circuit.
In order to output a signal input from terminal 1 with a maximum gain, in other words with a minimum attenuation, to terminal 2, the resistance value of bias resistor R3 is lowered and the value of current passed through diode D1 is increased for the purpose of reducing the attenuation of the signal and improving the noise figure.
Thus, the high frequency resistance of diode D1 decreases. However, if the resistance value of bias resistor R3 is extremely reduced, the impedance of the AGC circuit is lowered as a result, which conversely increases the noise figure.
The resistance value of bias resistor R3 should be set taking into account the balance between improvement of the noise figure due to current increase and the increase of the noise figure due to lowered impedance. Therefore, in the AGC circuit as shown in FIG. 7, for example, the resistance value of bias resister R3 is about several k .OMEGA., in other words it should be set to a relatively large value.
More specifically, in the conventional AGC circuit as shown in FIG. 7, even for the maximum level of the AGC signal, the high frequency resistance of diode D1 is not sufficiently lowered, and therefore the noise figure increases.
In order to restrict the noise figure from increasing as much as possible, the resistance value of bias resistor R3 should be reduced so that more current may be passed through diode D1. However, if the resistance value of bias resistor R3 is too small, the impedance of the circuit conversely decreases, which increases the noise figure.